Abstract : Complete vibrational assignments were made for a monosubstituted benzene with an electron-donating substituent (phenol) and one with an electronattracting substituent (benzonitrile). In order to confirm the assignments and to obtain more information on the modes of vibration, ringdeuterated aromatics, phenol and benzonitrile, were prepared and assigned. The effects of disubstitution were studied by making an assignment of p-cresol and several of its deuterated derivatives. By use of the results of these assignments and those available in the literature, frequency assignments in the CsBr spectral region (650-300 reciprocal centimeters) were made from the spectra of 279 benzene derivatives. These assignments include several benzene-like ring deformations and some vibrations that are either internal vibrations of the substituent group or involve the ring-substituent bond. The assigned frequencies show that group frequencies (or rather group perturbations) exist for benzene derivatives in the low-frequency region. Some data have been obtained on the low-frequency (300-50 cm -reciprocal centimeters) infrared spectra of phenol and p-cresol. From these data, assignments can be tentatively made for the O-H...O stretching and bending vibrations. (Author)